Or Blue-headed Finch. Blue-headed Vireos continue their fall migration later into the season than any other vireo. The occasional one shows up on a CBC. We're past the point where they're a daily occurrence, but they remain a real possibility. So if you're out and hear well-spaced rather musical phrases high in a tree you've got a Blue-headed Vireo, right. Vireos are noted for singing into fall. You might even utter the phrase most detrimental to pinning down an uncertain ID - "What else could it have been." (The reason to avoid this phrase at all costs is fodder for another post.)
There's another possibility, and right now probably the more likely one. Purple Finches in fall often sing a truncated version of their warbled song. A full Purple Finch song and a full Blue-headed Vireo song shouldn't be confused, but those truncated finch songs often come out as brief, musical, well spaced phrases, very much like a vireo. I heard one this a.m. in Naugatuck S.F. I started looking for the bird to be sure it was, as I suspected, a Purple Finch. but I never had to get a look. Soon the song phrases were interspersed with the very distinctive pick-pick call notes of a Purple Finch
Greg HanisekWaterbury